When X-rays are transmitted through a subject, the phase of the X-rays is changed due to the constituent elements and the difference in density of the subject. The research of the phase contrast imaging method using the phase change of the X-rays has been conducted since the 1990s. One of the phase contrast imaging methods is described in Patent Document 1. The method is a kind of a method referred to as a phase stepping method. The phase stepping method is featured in that the method is influenced by neither the transmittance distribution of the subject nor the spatial ununiformity of the intensity of X-rays irradiated onto the subject and has a high spatial resolution which is in principle the same as the resolution of an imaging element used to capture the image of the X-rays.
Patent Document 1 proposes an X-ray imaging apparatus as described below. In the apparatus, X-rays transmitted through a subject are irradiated onto a diffraction grating so that a periodic interference pattern referred to as a self-image is formed at a position away from the diffraction grating by a specific distance (Talbot distance). Further, in the apparatus, in order to generate a moiré, a masking grating having the same period as the period of the interference pattern is provided at the position where the interference pattern is formed, and the generated moiré is detected by an X-ray detector. Further, the apparatus is configured such that the phase information of the X-rays transmitted through the subject is acquired on the basis of three or more moiré images detected by moving the diffraction grating or the masking grating at a constant interval, and such that a phase contrast image of the subject is obtained from the phase information.